In many types of aircraft, particularly fixed wing aircraft, it is standard practice to avoid flying, under aerodynamically generated lift, at velocities close to stall airspeeds. This practice is even more strictly adhered to in the case for unmanned air vehicles (UAV), and improves safety and minimizes risk of loss or damage to the air vehicles, which can occur when losing aerodynamic control thereof.
However, this practice also results in a restriction in the operation of the air vehicle, particularly UAV's, to above a specified airspeed (which includes a speed safety margin imposed on stall airspeed), reducing operation in an otherwise allowable part of the flight envelope. The limitation is especially relevant for the flight and take-off/landing phases of small and medium size UAV flying at reduced airspeeds in windy air, and is also applicable to other aircraft For such cases, reducing the specified airspeed (which may be accompanied by an increased angle of attack to maximize lift) eventually leads to stall of the wings and loss of aileron's aerodynamically generated controlling forces, and may produce uncontrollable dynamic response of the aircraft, leading to the development of spin modes that are difficult to recover from, particularly in the case of unmanned air vehicles. In particular, there is often a fast variation in aerodynamic characteristics of the aircraft at stall and post-stall angles of attack that are highly unstable/unreliable and are difficult to predict, and which render the aircraft (specially UAV's) difficult to control in flight. The actual speed safety margin (to avoid the stall flight regime) imposed on aircraft operation is often dependent on stall pattern of the wing and may vary, depending on specific case and required flight conditions, and the standard speed safety margin adopted for UAV operations is: Vflight≧1.2 Vstall.
Stall characteristics for subsonic-type wings or aerofoils may be classified as follows:                Abrupt stall is characterized by the fast drop of the lift at stall angle of attack, typically resulting in an approximate 20-50% loss of lift coefficient within about 1° to about 3° after the stall angle of attack, depending on the level of maximum lift (e.g., moderate or high lift wings). This type of stall is associated with flow separation at the leading edge of the wing (leading edge stall), or, with very fast progress of trailing edge separation        Moderate stall is characterized by the gradual development of trailing edge separation at the stall of the wing and moderate loss of the lift at post-stall angles of attack, typically resulting in a rate of loss of lift coefficient at post-stall angles of attack that is approximately close to the rate or slope of lift coefficient gain prior to the stall angle of attack, but of opposite slope thereto. This type of stall is associated with moderate progress of trailing edge separation at post-stall angles of attack.        Mild stall is characterized by almost constant level of the lift at post-stall domain and is associated with slowly creeping trailing edge separation that moderates the rate of lift losses at high angles of attack, typically resulting in an approximately constant lift coefficient (within about 10% of the maximum lift coefficient for at least about 5° after the stall angle of attack).        
The stall angle of attack may be defined as the angle of attack at which maximum lift coefficient (or up to about 99% of maximum lift coefficient) is first realized. The stalling speed is dependent on the weight (W) of the air vehicle, maximum lift coefficient (Cl max), wing area (Sw), and air density (ρ), and is generally defined asVstall=(2W/(ρ*CLmax*Sw))0.5
These three types of stall are diagrammatically illustrated in FIG. 1 as curves A, B and C, respectively.
By way of example, two conventional mild stall airfoils FX61-184 and NACA-4415. i.e., known aerofoils having mild stall characteristics, are illustrated in FIGS. 4(a) and 4(b), and their lift coefficients are compared in FIG. 5(a). The possible effects of variation of camber and thickness in maximum lift of the NACA-4415 aerofoil are illustrated in FIG. 5(b) (i.e., in comparison with NACA aerofoils 6415 and 6418, respectively).
Stall characteristics also tend to deteriorate as maximum lift is increased, resulting in more difficulties when attempting to comply with considerations of flight safety and to avoid unfavorable stall patterns. This is especially relevant for high-lift, long endurance wings of some UAV, such as for example the HERON high-lift long endurance UAV, manufactured by Israel Aircraft Industries, Israel. In the HERON, optimum endurance performance is via high loitering lift coefficients, which requires high maximum lift.
FIGS. 3(a) and 3(b), illustrate some variations of aircraft endurance factor (CL1.5/CD) for the different levels of configuration parasite drag, and marks a general region therein generally relevant for UAV with high and moderate aspect ratio wings. For some cases of practical interest (for example, where there is increased parasite drag due to external installations, protruding payloads, engine-airframe integration, installation of cooling systems, etc.), there is a clear advantage of high-lift, loitering flight. However, with the need to impose a speed safety margin on the minimum loiter velocity Vloiter=1.2Vstall, the full potential for increased lift that may be generated when flying at lower velocities may not be achieved for such vehicle, in some state of the art air vehicles, particularly UAV's.